Investigation of biophysical mechanisms in gold nanoparticle mediated laser manipulation of cells using a multimodal holographic and fluorescence imaging setup

Optical Microscopy Systems for the Detection of Unlabeled Nanoparticles

Label-free detection of nanoparticles is essential for a thorough evaluation of their cellular effects. In particular, nanoparticles intended for medical applications must be carefully analyzed in terms of their interactions with cells, tissues, and organs. Since the labeling causes a strong change in the physicochemical properties and thus also alters the interactions of the particles with the surrounding tissue, the use of fluorescently labeled particles is inadequate to characterize the effects of unlabeled particles. Further, labeling may affect cellular uptake and biocompatibility of nanoparticles. Thus, label-free techniques have been recently developed and implemented to ensure a reliable characterization of nanoparticles.

This review provides an overview of frequently used label-free visualization techniques and highlights recent studies on the development and usage of microscopy systems based on reflectance, darkfield, differential interference contrast, optical coherence, photothermal, holographic, photoacoustic, total internal reflection, surface plasmon resonance, Rayleigh light scattering, hyperspectral and reflectance structured illumination imaging. Using these imaging modalities, there is a strong enhancement in the reliability of experiments concerning cellular uptake and biocompatibility of nanoparticles, which is crucial for preclinical evaluations and future medical applications.

The cellular response to plasma membrane disruption for nanomaterial delivery

Delivery of nanomaterials into cells is of interest for fundamental cell biological research as well as for therapeutic and diagnostic purposes. One way of doing so is by physically disrupting the plasma membrane (PM). Several methods that exploit electrical, mechanical or optical cues have been conceived to temporarily disrupt the PM for intracellular delivery, with variable effects on cell viability. However, apart from acute cytotoxicity, subtler effects on cell physiology may occur as well. Their nature and timing vary with the severity of the insult and the efficiency of repair, but some may provoke permanent phenotypic alterations. With the growing palette of nanoscale delivery methods and applications, comes a need for an in-depth understanding of this cellular response. In this review, we summarize current knowledge about the chronology of cellular events that take place upon PM injury inflicted by different delivery methods. We also elaborate on their significance for cell homeostasis and cell fate. Based on the crucial nodes that govern cell fitness and functionality, we give directions for fine-tuning nano-delivery conditions.

Transient nuclear lamin A/C accretion aids in recovery from vapor nanobubble-induced permeabilisation of the plasma membrane

Vapor nanobubble (VNB) photoporation is a physical method for intracellular delivery that has gained significant interest in the past decade. It has successfully been used to introduce molecular cargo of diverse nature into different cell types with high throughput and minimal cytotoxicity. For translational purposes, it is important to understand whether and how photoporation affects cell homeostasis. To obtain a comprehensive view on the transcriptional rewiring that takes place after VNB photoporation, we performed a longitudinal shotgun RNA-sequencing experiment. Six hours after photoporation, we found a marked upregulation of LMNA transcripts as well as their protein products, the A-type lamins. At the same time point, we observed a significant increase in several heterochromatin marks, suggesting a global stiffening of the nucleus. These molecular features vanished 24 h after photoporation. Since VNB-induced chromatin condensation was prolonged in LMNA knockout cells, A-type lamins may be required for restoring the nucleus to its original state. Selective depletion of A-type lamins reduced cell viability after VNB photoporation, while pharmacological stimulation of LMNA transcription increased the percentage of successfully transfected cells that survived after photoporation. Therefore, our results suggest that cells respond to VNB photoporation by temporary upregulation of A-type lamins to facilitate their recovery.

Stimulation of Local Cytosolic Calcium Release by Photothermal Heating for Studying Intra- and Intercellular Calcium Waves

A methodology is described that allows for localized Ca2+ release by photoexcitation. For this, cells are loaded with polymer capsules with integrated plasmonic nanoparticles, which reside in endo-lysosomes. The micrometer-sized capsules can be individually excited by near-infrared light from a light pointer, causing photothermal heating, upon which there is a rise in the free cytosolic Ca2+ concentration ([Ca2+ ]i ). The [Ca2+ ]i can be analyzed with a Ca2+ indicator fluorophore. In this way, it is possible to excite local lysosomal Ca2+ release in a desired target cell.

Single point single-cell nanoparticle mediated pulsed laser optoporation

This article presents an optical platform for studying the dynamics of nanoparticle assisted pulsed laser optoporation of individual living cells. Here plasmonic nanoparticles (NPs) act as markers of the exact spatial position of living cell membranes and as an enhancer for localized pulsed laser perforation. High contrast NP imaging using reflected light microscopy (RLM) allows accurate and automatic laser targeting at individual NPs for spatially controlled laser optoporation of single cells at a single point. The NP imaging method is compatible with fluorescence microscopy and a cellular incubator that allows study of real-time perforation kinetics of live cells and the optomechanical interaction of NPs with membranes. These parameters are of great interest for the development and experimental implementation of the technology of pulsed laser optoporation and transfection applied to single living cells as well as to bulk-level assays.

Transient Photoinactivation of Cell Membrane Protein Activity without Genetic Modification by Molecular Hyperthermia

Precise manipulation of protein activity in living systems has broad applications in biomedical sciences. However, it is challenging to use light to manipulate protein activity in living systems without genetic modification. Here, we report a technique to optically switch off protein activity in living cells with high spatiotemporal resolution, referred to as molecular hyperthermia (MH). MH is based on the nanoscale-confined heating of plasmonic gold nanoparticles by short laser pulses to unfold and photoinactivate targeted proteins of interest. First, we show that protease-activated receptor 2 (PAR2), a G-protein-coupled receptor and an important pathway that leads to pain sensitization, can be photoinactivated in situ by MH without compromising cell proliferation. PAR2 activity can be switched off in laser-targeted cells without affecting surrounding cells. Furthermore, we demonstrate the molecular specificity of MH by inactivating PAR2 while leaving other receptors intact. Second, we demonstrate that the photoinactivation of a tight junction protein in brain endothelial monolayers leads to a reversible blood-brain barrier opening in vitro. Lastly, the protein inactivation by MH is below the nanobubble generation threshold and thus is predominantly due to the nanoscale heating. MH is distinct from traditional hyperthermia (that induces global tissue heating) in both its time and length scales: nanoseconds versus seconds, nanometers versus millimeters. Our results demonstrate that MH enables selective and remote manipulation of protein activity and cellular behavior without genetic modification.

Gold nanoparticle-mediated laser stimulation induces a complex stress response in neuronal cells

Stimulation of neuronal cells generally resorts to electric signals. Recent advances in laser-based stimulation methods could present an alternative with superior spatiotemporal resolution. The avoidance of electronic crosstalk makes these methods attractive for in vivo therapeutic application. In particular, nano-mediators, such as gold nanoparticles, can be used to transfer the energy from a laser pulse to the cell membrane and subsequently activate excitable cells. Although the underlying mechanisms of neuronal activation have been widely unraveled, the overall effect on the targeted cell is not understood. Little is known about the physiological and pathophysiological impact of a laser pulse targeted onto nanoabsorbers on the cell membrane. Here, we analyzed the reaction of the neuronal murine cell line Neuro-2A and murine primary cortical neurons to gold nanoparticle mediated laser stimulation. Our study reveals a severe, complex and cell-type independent stress response after laser irradiation, emphasizing the need for a thorough assessment of this approach’s efficacy and safety.

Analysis of poration-induced changes in cells from laser-activated plasmonic substrates

Laser-exposed plasmonic substrates permeabilize the plasma membrane of cells when in close contact to deliver cell-impermeable cargo. While studies have determined the cargo delivery efficiency and viability of laser-exposed plasmonic substrates, morphological changes in a cell have not been quantified. We porated myoblast C2C12 cells on a plasmonic pyramid array using a 532-nm laser with 850-ps pulse length and time-lapse fluorescence imaging to quantify cellular changes. We obtain a poration efficiency of 80%, viability of 90%, and a pore radius of 20 nm. We quantified area changes in the plasma membrane attached to the substrate (10% decrease), nucleus (5 - 10% decrease), and cytoplasm (5 - 10% decrease) over 1 h after laser treatment. Cytoskeleton fibers show a change of 50% in the alignment, or coherency, of fibers, which stabilizes after 10 mins. We investigate structural and morphological changes due to the poration process to enable the safe development of this technique for therapeutic applications.

Modulation of cardiomyocyte activity using pulsed laser irradiated gold nanoparticles

Can photothermal gold nanoparticle mediated laser manipulation be applied to induce cardiac contraction? Based on our previous work, we present a novel concept of cell stimulation. A 532 nm picosecond laser was employed to heat gold nanoparticles on cardiomyocytes. This leads to calcium oscillations in the HL-1 cardiomyocyte cell line. As calcium is connected to the contractility, we aimed to alter the contraction rate of native and stem cell derived cardiomyocytes. A contraction rate increase was particularly observed in calcium containing buffer with neonatal rat cardiomyocytes. Consequently, the study provides conceptual ideas for a light based, nanoparticle mediated stimulation system.

Towards Effective Photothermal/Photodynamic Treatment Using Plasmonic Gold Nanoparticles

Gold nanoparticles (AuNPs) of different size and shape are widely used as photosensitizers for cancer diagnostics and plasmonic photothermal (PPT)/photodynamic (PDT) therapy, as nanocarriers for drug delivery and laser-mediated pathogen killing, even the underlying mechanisms of treatment effects remain poorly understood. There is a need in analyzing and improving the ways to increase accumulation of AuNP in tumors and other crucial steps in interaction of AuNPs with laser light and tissues. In this review, we summarize our recent theoretical, experimental, and pre-clinical results on light activated interaction of AuNPs with tissues and cells. Specifically, we discuss a combined PPT/PDT treatment of tumors and killing of pathogen bacteria with gold-based nanocomposites and atomic clusters, cell optoporation, and theoretical simulations of nanoparticle-mediated laser heating of tissues and cells.

Biodegradable microsphere-mediated cell perforation in microfluidic channel using femtosecond laser

The use of small particles has expanded the capability of ultrashort pulsed laser optoinjection technology toward simultaneous treatment of multiple cells. The microfluidic platform is one of the attractive systems that has obtained synergy with laser-based technology for cell manipulation, including optoinjection. We have demonstrated the delivery of molecules into suspended-flowing cells in a microfluidic channel by using biodegradable polymer microspheres and a near-infrared femtosecond laser pulse. The use of polylactic-co-glycolic acid microspheres realized not only a higher optoinjection ratio compared to that with polylactic acid microspheres but also avoids optical damage to the microfluidic chip, which is attributable to its higher optical intensity enhancement at the localized spot under a microsphere. Interestingly, optoinjection ratios to nucleus showed a difference for adhered cells and suspended cells. The use of biodegradable polymer microspheres provides high throughput optoinjection; i.e., multiple cells can be treated in a short time, which is promising for various applications in cell analysis, drug delivery, and ex vivo gene transfection to bone marrow cells and stem cells without concerns about residual microspheres.

Tile-Based Two-Dimensional Phase Unwrapping for Digital Holography Using a Modular Framework

A variety of physical and biomedical imaging techniques, such as digital holography, interferometric synthetic aperture radar (InSAR), or magnetic resonance imaging (MRI) enable measurement of the phase of a physical quantity additionally to its amplitude. However, the phase can commonly only be measured modulo 2π, as a so called wrapped phase map. Phase unwrapping is the process of obtaining the underlying physical phase map from the wrapped phase. Tile-based phase unwrapping algorithms operate by first tessellating the phase map, then unwrapping individual tiles, and finally merging them to a continuous phase map. They can be implemented computationally efficiently and are robust to noise. However, they are prone to failure in the presence of phase residues or erroneous unwraps of single tiles. We tried to overcome these shortcomings by creating novel tile unwrapping and merging algorithms as well as creating a framework that allows to combine them in modular fashion. To increase the robustness of the tile unwrapping step, we implemented a model-based algorithm that makes efficient use of linear algebra to unwrap individual tiles. Furthermore, we adapted an established pixel-based unwrapping algorithm to create a quality guided tile merger. These original algorithms as well as previously existing ones were implemented in a modular phase unwrapping C++ framework. By examining different combinations of unwrapping and merging algorithms we compared our method to existing approaches. We could show that the appropriate choice of unwrapping and merging algorithms can significantly improve the unwrapped result in the presence of phase residues and noise. Beyond that, our modular framework allows for efficient design and test of new tile-based phase unwrapping algorithms. The software developed in this study is freely available.

Characterization of the cellular response triggered by gold nanoparticle-mediated laser manipulation

Laser-based transfection techniques have proven high applicability in several cell biologic applications. The delivery of different molecules using these techniques has been extensively investigated. In particular, new high-throughput approaches such as gold nanoparticle–mediated laser transfection allow efficient delivery of antisense molecules or proteins into cells preserving high cell viabilities. However, the cellular response to the perforation procedure is not well understood. We herein analyzed the perforation kinetics of single cells during resonant gold nanoparticle–mediated laser manipulation with an 850-ps laser system at a wavelength of 532 nm. Inflow velocity of propidium iodide into manipulated cells reached a maximum within a few seconds. Experiments based on the inflow of FM4-64 indicated that the membrane remains permeable for a few minutes for small molecules. To further characterize the cellular response postmanipulation, we analyzed levels of oxidative heat or general stress. Although we observed an increased formation of reactive oxygen species by an increase of dichlorofluorescein fluorescence, heat shock protein 70 was not upregulated in laser-treated cells. Additionally, no evidence of stress granule formation was visible by immunofluorescence staining. The data provided in this study help to identify the cellular reactions to gold nanoparticle–mediated laser manipulation.